Contact support, I’d say. ISTM there is a hardware issue on the FW mainboard side. Maybe a video’d test like mine above will convince them to skip all the standard steps that turned out to be useless.
I am having similar issues on NixOS 24.11 running hyprland. I have a DP monitor in the rear-right port and a HDMI monitor in the front-right port.
Everything is fine if I boot with the monitors connected and leave them connected. Suspend and hibernate work just fine. On resume, the monitors both come on.
But if I suspend or hibernate with the monitors connected and disconnect one or both monitors, on resume hyprland crashes and I get the “i915 ERROR Link Training Unsuccessful”.
I have tried different ports and they all produce the same results.
Edit: Kernel 6.10.6. Both modules are the latest versions.